Semiconductor laser device and circuit for and method of driving same

ABSTRACT

A directly driven laser includes multiple contacts, with at least one of the contacts for injecting current into the laser such that the laser reaches at least a lasing threshold and at least one of the contacts for providing a data signal to the laser. In some embodiments a differential data signal is effectively provided to a front and a rear section of the laser, while lasing threshold current is provided to a central portion of the laser.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Application No. 61/163,748, filed Mar. 26, 2009, entitled “ASemiconductor Laser Device”, and U.S. Provisional Application No.61/168,190, filed Apr. 9, 2009, entitled “A Method For DirectlyModulating Lasers”; the disclosures of which are incorporated byreference.

BACKGROUND OF THE INVENTION

Optical fiber communications has generally replaced electrical linksover long distances in the past few decades. In more recent past,optical links are being used at shorter distances to connect servers toswitches and for datacenters.

The advantages of fiber optics compared to electrical links are thegreater bandwidth and reduced degradation of the signal with distance.At 10 Gb/s data rates, for the signal to travel more than 100-300 m in afiber, generally single mode fiber is needed, with a typical mode sizeof about 8 microns. As an alternative, when distances are on the orderof 100 m or less, multimode fiber and multimode vertical cavity lasersmay be used. In this case the core size in the fiber is much larger atabout 50 um, and alignment tolerances can be substantially looser.However, the reach is limited as different modes of the fiber travel atdifferent speeds and it is becomes more difficult to transmit multiplewavelength simultaneously.

As bandwidth requirements increase, there is increased parallelism inboth single mode and multimode fiber links. In multimode systems,additional fibers may be added to form a fiber ribbon. One greatadvantage of single mode fiber is that multiple wavelengths can becoupled simultaneously to get a parallel link through a single fiber.Thus a 100 Gb/s signal can be sent through a single mode fiber for manykilometers by using ten channels of 10 Gb/s each, with every lane at adifferent wavelength. For multimode applications, 120 Gb/s may betransmitted over 100 m using a 12 element array of vertical cavitylasers coupled to ribbon fiber with 12 fibers for transmit and 12 forreceive. The parallel ribbon fibers are of course quite expensive andconnectors with 24 fibers inside are complicated to make, even if theyuse multimode fiber with looser alignment tolerance.

Generally it is desirable to minimize the electrical power consumed andhence the heat generated by any optical link, because of the desire topack the electronics and optics into as compact a space as possible.Therefore it is generally preferred to minimize the electrical drivecurrent for each laser. Vertical cavity surface-emitting lasers(VCSELs), with their very small active area, have a threshold currentfor lasing typically below 5 milliamps, and can operate at a biascurrent below 20 milliamps, with a peak-peak modulation current of 10milliamps or less. These lasers, however, have serious drawbacks inmultiple wavelength links. Their lasing wavelengths generally scalelinearly with the thickness of the semiconductor layers that make up thelaser cavity, and generally cannot be controlled to within the accuracyrequired in multiple wavelength systems, which is typically 0.1% orbetter. Also their small area and circular shape give them a highthermal resistance, and their output optical power is generally limitedto below 1 milliwatt, especially at high temperature. The opticalelements that combine optical signals at different wavelengths into asingle fiber inevitably introduce optical loss. In order to ensure thatsufficient optical power is transmitted for reliable data transfer,there is generally a minimum output power requirement for the laser,typically in excess of 1 milliwatt.

Greater optical power output and wavelength control can often beprovided by edge-emitting mode-controlled semiconductor lasers such asdistributed feedback (DFB) lasers. In these lasers, the wavelength iscontrolled by the periodicity of an etched grating, which is controlledvery precisely by lithography, either optical or electron beam. Thenarrow stripe geometry of DFB lasers is very suitable for heatdissipation, so these lasers can be driven at high current in order toachieve high optical power output. The disadvantage of conventional DFBlasers is they generally use a high electrical current drive relative toVCSELs. The threshold current for lasing of DFBs is typically around 10milliamps at 25° C. and 25 to 40 milliamps at 85° C.

Unfortunately it is difficult to achieve very low threshold current fora DFB laser, for example by reducing the length of the laser, becausethe required optical gain per unit length increases beyond what iseasily achievable. Another problem with shorter DFB lasers of forexample 110 μm length is that it is difficult to cleave devices shorterthan about 200 μm.

Regarding possible use of Distributed Bragg Reflector (DBR) laserdesigns, the disadvantages of this structure relate to the longitudinaloptical mode control, since there is no grating in the active region.This type of laser has a certain yield for single-wavelength operation,which means that screening is required. A bigger problem is the tendencyto “hop” from one wavelength to another as the drive current is changed.This tendency means that there would be a narrow range of acceptablebias current for any individual laser, so device yield would be quitelow and testing and calibration would be time consuming.

The electrical drive circuitry that supplies current to the laser diodeis another cause of undesired power dissipation, because of itscomplexity and the general requirement to provide separate current pathsfor the direct current bias and the radio-frequency (RF) data signal.The simplest method to modulate a DML for the transmission of binarydata is to turn the laser on for transmission of a 1 bit and to turn thelaser off for the transmission of a 0 bit. This method only works wellfor relatively low bit rates (up to Mb/s) as the turn on delay of thelaser and associated noise and laser response (laser relaxationoscillation) result in significant degradation of the transmitted signalfor bit rates exceeding several Mb/s. Therefore at higher bit rates itis necessary to keep the laser above its lasing threshold condition atall times, and hence the laser is modulated from a low lasing power P₀,achieved at laser current I₀, for a 0 bit, to a high lasing power P₁,achieved at a laser current I₁, for a 1 bit, as illustrated in FIG. 1.Laser drivers for DMLs at bit rates exceeding several Mb/s typicallyconsist of a current steering output stage and therefore can driveeither zero current for a 0 bit or a nonzero modulation current for a 1bit. As a result the nonzero current I₀ for a 0 bit has to be providedthrough a different path to the laser. This DC path is typicallyimplemented using inductors to provide high impedance as to block the RFmodulation from leaking out through this path, yet be low impedance atDC due to the limited DC supply voltage available. For optical links theRF content of the data covers a wide frequency spectrum resulting in aninductive path that typically consists of several low cost inductors andresistors, or costly high quality inductors. In addition the voltagelevel for the laser typically does not match the voltage levels for theoutput stage of the laser driver and as a result the laser driver andlaser are AC coupled using a capacitor, and additional inductors areplaced on the laser driver side of the AC coupling capacitor to providethe correct DC voltage level on the driver output. For 10 Gb/s this canlead to 10 to 25 passive components in the circuit connecting the laserdriver output to the laser as shown in FIG. 2 which shows a directlymodulated laser diode 21 driven with a data signal provided by a driver23, with passive components 25, for example as mentioned above. Some ofthose components can be of significant size to provide high impedance atrelatively low frequencies. This typical bias circuit, usually referredto as a bias tee, is not only large in size but also inefficient interms of electrical power dissipation, as the DC component of the LDDoutput stage is not used in the LD due to the DC blocking of thecoupling capacitors. In some implementations the differentialconfiguration shown in FIG. 2 is simplified to a single endedconfiguration, resulting in a lower number of passive components. Forapplications with multiple 10 Gb/s lanes in the same module, however, adifferential configuration would be preferred to reduce link performancedegradation caused by electrical crosstalk between the lanes. Variationson this, which add a laser driver final stage co-located in the laserpackage, use relatively large components inside the laser package, whichis undesirable as well.

BRIEF SUMMARY OF THE INVENTION

In one aspect the invention provides a distributed feedback (DFB) laser,comprising: a waveguide including an active region, with a grating in orabout the waveguide, the waveguide having a longitudinal length in adirection between a front facet and a rear facet, the waveguide beinggenerally about layers forming a p-n junction, and an anode electricalcontact and a cathode electrical contact; with at least one of the anodeelectrical contact and the cathode electrical contact having alongitudinal length in the direction between the front facet and therear facet less than the longitudinal length of the active region.

In another aspect the invention provides a method of operating a laser,comprising: providing a data current signal into a first longitudinalportion of the laser and not providing the data current signal into asecond longitudinal portion of the laser; and providing a bias currentsignal into the second longitudinal portion of the laser and notproviding the bias current signal into the first longitudinal portion ofthe laser.

In another aspect the invention provides a method of operating a laser,comprising: providing a differential signal to a pair of gain stageswithin a package containing the laser, the differential signal providinga data signal, the pair of gain stages providing a signal to a frontportion of the laser and/or a rear portion of the laser depending onstate of the differential signal; and providing a bias signal to acentral portion of the laser.

In another aspect the invention provides a laser and drive circuitry,comprising: a laser with a first electrical contact and a secondelectrical contact, each for provision of current signals to the laser,and a contact coupled to ground; a dc current source coupled to thefirst electrical contact; and a data signal driver coupled to the secondelectrical contact.

Some aspects of the invention provide a directly modulated distributedfeedback laser where there are at least two sections to the laser andwhere modulated current injection is confined to the section with thehighest optical mode density.

In some such aspects there is a phase shift in the grating in thecavity, the two facets of the laser cavity are anti-reflection coated,and modulated current injection is limited to the section of the laseraround the phase shift region and towards the front of the laser.

In some such aspects the other section of the laser is also electricallypumped but only with a DC current to maintain transparency. The gainmaterial is rendered non-absorbing outside of the pumping region bybeing physically removed through an etch and replaced by a higherbandgap semiconductor material or by being composed of material ofhigher bandgap deposited through selective area growth or by beingdisordered and diffused into a higher bandgap state through acombination of an implant and an anneal.

In some such aspects there is no phase shift in grating providingfeedback in the cavity and where the rear of the laser is coated with ahigh reflectivity coating and the front is coated with ananti-reflectivity material and where the rear of the laser iselectrically pumped with the modulation signal and the front region iseither unpumped or made optically transparent with a DC current.

In some such aspects there are three sections to the laser and wherethere is a phase shift in the cavity, where the optical mode peaks. Onlythe more central section that contains the phase shift region is pumpedwith a modulation current and where the two other sections in the veryfront and the rear of the laser are left either unpumped, or fed with aDC current to render them transparent.

Some aspects of the invention provide a laser diode supplying sufficientoptical power for multiple-wavelength optical transmission systems, withexcellent wavelength control, while operating at a low enough electricaldrive current for it to be driven by inexpensive low-power electronics.

In aspects of the invention a distributed feedback laser is provided, inwhich the electrical drive current is applied to a limited section ofthe device. In some embodiments electrical drive current is applied to alimited section of the device for the purpose of lowering the electricaldrive current. In some embodiments the remainder of the device eitherhas no electrical current drive or a continuous current drive, andprovides optical feedback.

One aspect of the invention provides a laser diode and associated biasand modulation circuit that can be connected to the laser diode driverwith a significantly reduced number of passive components. In anotheraspect the invention provides a directly modulated laser in which the DCbias is applied to one section of the laser and the high-speedmodulation signal is applied to a different section of the laser, whichin some aspects eliminates costly and bulky bias tees. Some of theembodiments of the invention also achieve a reduction in electricalpower dissipation by eliminating the AC coupling between the laser diodedriver and the laser diode.

In some aspects of the invention, a multi-section directly modulatedlaser (DFB or FP) is provided, in which the DC electrical drive currentis applied to one or more sections of the device, for the purpose ofbiasing the laser above its lasing threshold and avoid signaldegradation from laser turn on. The RF electrical modulation currentcontaining the data is applied to one or more different sections withoutany DC prebias. The RF electrical modulation current on those sectionsmodulates the laser optical output power without any turn on relatedsignal degradation (for example jitter or ringing) as the laser isbiased above threshold by means of the DC electrical drive current inthe first section. In some embodiments this method of directlymodulating a laser eliminates the need of a large number of passiveelectrical components in the circuit connecting the laser diode driverto the laser diode. In embodiments where AC coupling is eliminated, forexample, a reduction in electrical power dissipation is obtained aswell. For certain embodiments of the invention, the modulation currentresults in a larger optical modulation amplitude out of one of the laserfacets, compared to applying, the same total current modulation to asingle section device, at the expense of smaller optical modulationamplitude out of the other facet. This leads to an additional reductionin electrical power dissipation as for the same optical modulationamplitude less electrical modulation current is needed

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing laser optical power versus drive current;

FIG. 2 is a semi-schematic of a laser and drive circuitry;

FIG. 3 illustrates a DFB laser with a partial cutaway section;

FIG. 3 a illustrates a cross-section of the DFB laser of FIG. 3;

FIG. 4 illustrates a cross-section of a DFB laser in accordance withaspects of the invention;

FIG. 5 further illustrates the cross-section of a DFB laser of FIG. 4

FIG. 5 a is a chart of optical intensity versus position in the cavityof the laser of FIG. 5;

FIG. 6 is a chart of simulated output power versus drive current;

FIG. 7 is a semi-schematic and semi-cross section illustration of alaser and drive circuitry in accordance with aspects of the invention;

FIG. 8 illustrates a cross-section of a further DFB laser in accordancewith aspects of the invention;

FIG. 9 illustrates a cross-section of a further DFB laser in accordancewith aspects of the invention;

FIG. 10 illustrates a cross-section of a further DFB laser in accordancewith aspects of the invention;

FIG. 10 a is a chart of optical intensity versus position in the cavityof the laser of FIG. 10;

FIG. 11 is a semi-schematic and semi-cross section illustration of alaser and drive circuitry in accordance with aspects of the invention;

FIG. 12 is a chart of optical power versus laser current;

FIG. 13 is a chart of modulated optical power versus modulationfrequency;

FIG. 14 is a chart of optical power versus time;

FIG. 15 is a semi-schematic and semi-cross section illustration of alaser and drive circuitry in accordance with aspects of the invention;

FIG. 16 is a semi-schematic and semi-cross section illustration of alaser and drive circuitry in accordance with aspects of the invention;

FIG. 17 is a semi-schematic and semi-cross section illustration of alaser and drive circuitry in accordance with aspects of the invention;

FIG. 18 is a semi-schematic and semi-cross section illustration of alaser and drive circuitry in accordance with aspects of the invention;

FIG. 19 is a semi-schematic and semi-cross section illustration of alaser and drive circuitry in accordance with aspects of the invention;

DETAILED DESCRIPTION

Distributed feedback (DFB) semiconductor lasers have been the preferredoptical sources for fiber-optic transmission systems for several years.A typical DFB laser is illustrated in FIG. 3, while a cross-sectionalong the dotted line is shown in FIG. 3 a. Features of the illustratedDFB laser are as follows:

The DFB laser contains a waveguide structure 310 whose core providesoptical gain under current injection. Typically the gain materialcontains one or more quantum wells, semiconductor layers with thicknessbetween 2 nm and 20 nm. Typically the length of the waveguide is atleast 0.2 mm, although some examples are shorter. See, e.g., K. Nakaharaet al., “40-Gb/s Direct Modulation With High Extinction Ratio Operationof 1.3-μm InGaAlAs Multiquantum Well Ridge Waveguide DistributedFeedback Lasers”, IEEE Photonics Technology Letters Vol. 19, No. 19,Oct. 1 2007, pp. 1436-1438, incorporated by reference herein. Typicallythe end facets are formed by cleaving along crystal planes, althoughsome lasers have facets formed by etching.

A grating structure 320, running along the waveguide is etched into oneor more of the layers comprising the waveguide. This grating provideswavelength-selective optical feedback. In some cases one or more phaseshifts 330 are introduced into the grating structure in order to enhancethe wavelength selectivity. In the case that a phase shift isincorporated, and as illustrated in FIG. 3 a, the facets are generallycoated with anti-reflection coatings 340. In the case that the gratingcontains no phase shift, the rear facet is typically coated for highreflectivity, while the front facet is coated for low reflectivity. Thefront facet of the laser is generally understood in the art to be thefacet from which useful light is generally emitted, although it is alsogenerally understood in the art that light emitted from the rear facetmay also be utilized, for example for monitoring purposes.

The semiconductor structure generally forms a p-n diode, with thejunction located in or close to the gain region. An anode electricalcontact 350 (also referred to as a top electrical contact herein, and asshown in FIG. 3) is usually applied to the p region, while a cathodeelectrical contact 360 (also referred to as a bottom electrical contactherein, and as shown in FIG. 3) is made to the typically n-dopedsubstrate region.

The DFB in FIG. 3 is a buried heterostructure device. In contrast to asimpler ridge laser, where the lateral confinement is obtained by simplyetching a ridge into the laser (which may instead, for example, be doneand used), in a buried heterostructure, the ridge or mesa is regrownwith a blocking junction, thus surrounding the optical mode withsemiconductor on all sides. In FIG. 3 a, the laser includes layers 370and 380, which are the p and n blocking junctions respectively.

An embodiment of a cross-section of a DFB laser in accordance withaspects of the invention is illustrated in FIG. 4. FIG. 4 shows a DFBlaser, with a phase-shifted grating 410, a waveguide 420, a topelectrical contact 450, and a bottom electrical contact 460. Asillustrated in FIG. 4 the front section is the front half of the laserand the rear section is the rear half of the laser. As one of skill inthe art would understand, however, the front section, or alternativelythe rear section, is longer or shorter in various embodiments. The topelectrical contact 450 is placed only on a front section of the laser,thereby restricting electrical current injection to this region of thelaser. Of course, in various embodiments isolation implants or otherstructures may be used to restrict injection of current to particularregions of the laser. A rear section of the laser provides opticalfeedback, since the grating is present, but the rear section provides nooptical gain because there is no current injection in the rear section.If quantum well material is kept in the waveguide in the rear section,there will be some optical absorption in this region. At high opticalintensities, the material will bleach out and only present a minorinefficiency in the laser.

An advantage of this arrangement of the electrical contacts, compared toa device the same length but with the top electrical contact extendingthe entire length of the device, is that the electrical current togenerate electrical gain is reduced. With appropriate selection of theso-called “coupling strength”, a measure of the optical feedback perunit length provided by the grating, the threshold current and operatingcurrent can be reduced compared to conventional DFB device.

An alternative approach is to make a conventional DFB laser of a shortercavity length, corresponding to the length of the top electrical contactin the arrangement illustrated in FIG. 4, with a higher grating couplingstrength in order to provide the required optical feedback in a shortercavity length. There are several advantages to taking an approachdescribed herein, some of which will be applicable depending on theproposed application and on the established technology at the place ofdesign or manufacture.

In many embodiments a desired overall optical feedback, characterized bythe product of the grating coupling strength and the length of thegrating, can be achieved with a lower coupling strength. There arepractical technological limits to the coupling strength, which dependson (i) the depth and shape of the grating teeth, (ii) the differencebetween the refractive indices of the material in which the grating isetched and the material that fills in between the teeth, and (iii) howmuch of the optical waveguide mode is confined within the gratingstructure. For very short laser cavities it may be impossible to achievea desired coupling strength when the grating is required to be the samelength as the gain region.

Another advantage is that the optical gain is provided in the regionwhere the optical intensity is highest. FIG. 5A shows a curve 500 of theintensity of the light in a laser cavity, for example the laser of FIG.5, which as illustrated corresponds to the laser of FIG. 4. It is higherin the front of the laser than in the back (or rear) of the laser, andhas a maximum at the position of a phase shift 530 in the grating in thelaser. Generally the longitudinal design will be optimized for higheroutput power at the front facet than at the rear facet, for example bylocating the phase shift towards the front of the laser. Optical gain isgenerally used most efficiently when it occurs in regions where theoptical intensity is high. This principle was investigated with respectto VCSELs in which the length of the optical cavity is only one or a fewoptical wavelengths. The gain material is placed only at the peaks ofthe optical standing wave by careful design of the vertical structure ofa VCSEL. For a VCSEL this arrangement has been shown to increase theeffective optical gain, for example as described in S. W. Corzine etal., “Design of Fabry-Perot surface-emitting lasers with a periodic gainstructure”, IEEE J. Quantum Electronics vol. 25, no. 6, June 1989, p.1513-1524, the disclosure of which is incorporated by reference herein.In accordance with aspects of the invention, the location of the opticalgain along the cavity is determined by the electrical contacts ratherthan the vertical material structure. The optical intensity is quite lowtowards the rear facet of a phase-shifted DFB laser, so any currentinjected there is not used efficiently. Thus one can obtain maximumefficiency by injecting the current only in the front section, forexample trough use of a top electrical contact that is on a frontportion (front half as illustrated in FIG. 5 a) of the laser of the DFBwhere the optical mode is the strongest.

The modulation speed of the laser also depends on the size of the lasingcavity. The longer the laser and the photon residence time, the slowerthe modulation frequency. Thus it is preferred in laser designs such asshown in FIG. 5 to use the strongest grating strength possible tominimize the amount of light penetrating the back grating. There is notrade-off between low modulation current and high speed operation, asboth generally require a short cavity with high grating strengths.

To illustrate aspects of the invention, simulations have been performed.A detailed longitudinal model of a DFB laser cavity has been developed.The etched grating is analyzed using the coupled-mode equations, and theinteracting densities of photons and electrons are calculated using rateequations. The longitudinal cavity is divided into multiple shortsections, in each of which the photon and electron densities, gratingstrength, temperature, etc., are kept constant, but can vary fromsection to section. The local optical gain, absorption and refractiveindex are modeled parametrically as functions of electron density andtemperature, which is estimated based on separately calculated ormeasured thermal resistance. An iterative approach, based onmultiplication of characteristic matrices for each section, is used tosolve self-consistently for the electron density, optical power,temperature, effective refractive index and optical gain for eachsection at a given current.

Simulations were performed for a DFB laser 250 microns long, with agrating whose coupling strength is 100 inverse cm, with a phase shiftplaced 100 microns from the front facet. This grating strength isregarded as quite high using standard materials and technologies. Fourseparate cases are considered, and simulated curves of power vs currentare shown in FIG. 6: (i) current is applied over the entire length ofthe structure-shown as a first curve 620; (ii) current is applied overthe front 150 microns of the structure, with no current applied to therear 100 microns, where there is some optical absorption rather thangain—shown as a second curve 600; (iii) the device is shortened to 150microns long, with the same grating strength—shown as a third curve(430); (iv) the device is 150 microns long, with the grating strengthincreased to 190 inverse cm (in order to obtain the same thresholdcurrent as in case (ii))—shown as a fourth curve 610. A comparison ofthe different cases shows that case (ii), with current injection overonly part of the cavity, gives significantly lower threshold currentthan cases (i) and (iii). Comparison with case (iv), the shorter DFBlaser with a stronger grating that has a similar threshold current,shows that case (ii) offers higher output efficiency. A furtheradvantage of case (ii) over case (iv) is that case (iv) requires a muchhigher grating strength, most likely a deeper etch, pushing thetechnological boundaries and possibly introducing additional losses dueto light scattering that are not considered in this model.

There are many modifications that can be made that can further enhanceperformance of a DFB laser.

In some embodiments, the physical period of the grating may differbetween the two sections, in order to achieve the desired matching ofthe optical characteristics while taking into account differenttemperatures and electrical carrier densities in the two sections.

FIG. 7 shows a similar DFB laser to that in FIG. 4, but the laser ofFIG. 7 contains a separate contact 700 to the rear section of the laserin addition to the main laser anode contact 750. The laser is fedcurrent with two separate electrical lines. The electrical line feedingcurrent the rear of the laser, where the optical mode is weakest, isconnected to a DC current source 710. This current would be a low steadyvalue meant to keep the rear of the laser transparent. The main laseranode 750 is fed with a conventional electrical input of a combinationof a DC current 720 and an AC modulation current 730 that contains thesignal that is to be transmitted. Thus only one contact where theoptical mode is highest, the front contact 750, is connected to themodulation current. In this way the optical loss in the unpumped sectioncan be substantially eliminated by a small DC current, while the lowsignal drive current is well within the reach of inexpensiveelectronics.

The two sections, the front section and the rear section, preferably areelectrically isolated. This can be done by etching the top p-cladding,for example to form a trench partially into the laser—deep enough tosubstantially increase the electrical resistance between the twosections, but not so deep as to induce high optical loss, oralternatively to implant the region between the two sections withprotons or helium to increase resistance. In FIG. 7, an isolationimplant 740 is shown between the two electrical contacts.

FIG. 8 shows a similar DFB laser to that in FIG. 4, but in this case thegain region 810 has been etched away in the region without a contact andreplaced by a non-absorbing waveguide region 800. Again, the loss issubstantially eliminated from the rear section, and in this case noseparate DC current is required. A slight disadvantage of this approachis the higher process complexity, for example etching and regrowing apassive waveguide within the laser and also possibly additional loss inthe transition between the two waveguides.

Another technique to eliminate the active region 810 and the associatedloss from the rear of the laser is by using selective area growth. Inthis technique an oxide mask with windows is used on the wafer duringthe growth process. The rate of growth increases in the regions wherethere is a large area of oxide, and the quantum wells become thicker aswell as change in composition. One side of the laser can therefore bemade transparent with a wider bandgap by including oxide regions aroundthe active section.

A third technique to increase the bandgap on the rear of the laser andmake it transparent is with the use of an implant coupled with ananneal. In impurity induced disordering methods, the side of the laserto be made transparent is implanted with a material that causesvacancies. During an anneal these vacancies migrate and cause thequantum wells to wash out thereby increasing the bandgap.

In many bandgap changing techniques, such as etch and regrow, orselective area growth, it is advantageous to put the diffraction gratingbelow the active layer. In this way, the planarity of the grating layeris not disturbed by the etch and regrowth process of the active layer.Thus in FIG. 8, the grating layer 820 is shown below the active region800.

FIG. 9 shows a similar DFB laser to that in FIG. 8, but in this case thegrating is uniform with no phase shift section, and the rear facet has ahigh-reflectivity optical coating 900. In this case the current may beapplied to the rear section of the laser, since the optical intensity isstrongest there. The same techniques previously discussed can be appliedto make the active layer transparent on the side of the laser with thereduced optical field, and obtain the advantage of reduced operatingcurrent and higher speed modulation.

FIG. 10 shows a similar DFB laser to that in FIG. 8, but the DFB laserof FIG. 10 contains two non-absorbing regions 1011, 1013 in thewaveguide at the front and rear of the laser, with a top electricalcontact 1015 over a central active region 1017 of the waveguide. Thisarrangement takes further advantage of the optical intensitydistribution along the laser cavity (as shown in FIG. 10 a), placing thegain only where the intensity is highest. In addition, in someembodiments the coupling coefficient of the grating is modified comparedto the case illustrated in FIG. 3 a, in order to enhance the peaking ofthe optical intensity around the grating phase shift position and tolower the optical intensity near the front facet.

FIG. 11 shows a similar laser to that in FIG. 7, with modifiedelectrical drive circuitry that reduces complexity, electrical powerdissipation, and cost. In the embodiment of FIG. 11, a grating 1110 ofthe laser is uniform, a rear facet has a high reflectivity coating 1120and a front facet has a low reflectivity coating 1130. DC current isapplied to the rear section of the laser at a current level sufficientto cause the rear section to laser. For a 0 bit no current is applied tothe front section, such that the optical power emitted by the rearsection is partially absorbed by the front section, and a low level ofoptical power (P₀ in FIG. 1) is emitted out of the front facet. For a 1bit a nonzero current is applied to the front section of the laser. Thischanges the overall lasing condition of the device as the lasereffectively becomes longer, lowering the lasing threshold condition andincreasing the generation of light in the front section. The poweremitted out of the front facet of the laser is now significantly larger(P₁ in FIG. 1). As the laser was already lasing, and lasing power waspresent in the front section, there is no signal degradation associatedwith turning on the front section. This turn on degradation is caused bythe need of optical power to be initially generated by spontaneousemission, which is a slow and random process. In this embodiment theoptical power present in the front section substantially eliminates theneed for optical power to be generated by spontaneous emission. As bothsections are electrically isolated, there is no need to use bias tees.The rear section is directly connected to a DC current source 1140, andthe front section is directly connected to a laser driver 1150 thatprovides a modulated current between zero and a nonzero value through anRF matching resistor 1160.

To illustrate this aspect of the invention, simulations have beenperformed based on a well-known rate equation model of semiconductorlasers. The typical single section rate equations have been expanded totwo sections with an electron density and current injection for eachsection, as well as an optical power for each section and coupling ofthe optical powers between the two sections. A laser with a rear sectionof 0.1 mm length and a front section of 0.05 mm has been assumed.Simulated front facet optical power versus total DC current in the laser(rear+front current) is shown in FIG. 12 for four different cases ofdriving the laser. In case (i) the current in the front section is fixedto zero while the current in the rear section is varied. In case (ii)the current in the front section is fixed to 10 mA while the current inthe rear section is varied. In case (iii) the current in both sectionsare varied together proportionally as if both sections were combined asone, and in case (iv) the current in the rear section is fixed to 20 mAwhile the current in the front section is varied. Comparison betweencase (i) and case (ii) illustrates that a 10 mA increase in current inthe front section can more than double the optical power emitted by thefront facet. Comparison between case (iii) and case (iv) illustratesthat fixing the current in the rear section and modulating the currentin the front facet can result in higher slope efficiency out of thefront facet, compared to the single section laser. Hence to achieve therequired optical modulation amplitude out of the laser, less modulationcurrent is needed and less electrical power is dissipated.

To determine if this two-section laser has large enough modulationbandwidth despite having the front section modulated from zero current,the rate equation model was solved under modulation for the particularcase where the rear section is biased with a DC current of 20 mA. FromFIG. 12 it is seen that with a rear section at 20 mA DC current, achange in current in the front section from 0 to 10 mA will more thandouble the front facet optical power. The simulated small signalfrequency modulation response is shown in FIG. 13 for four differentcases. For case (i) the current in the rear section is fixed at 20 mA,and the front section is modulated around an average current of zero,while for case (ii) the rear section current is fixed at 20 mA while thefront section is modulated around an average current of 10 mA. Cases(iii) and (iv) correspond to the equivalent modulation of the singlesection device, with case (iii) at an combined average current of 20 mAand case (iv) an combined average current of 30 mA. Cases (i) and (ii)illustrate that the modulation bandwidth is high enough for 10 Gb/s. Therate equation model was used to simulate a large signal step responsewith the results shown in FIG. 14 for 2 cases. In case (i) the rearsection current is fixed at 20 mA and the front section is modulatedfrom 0 mA to 10 mA with a square wave with a period of 1.6 ns. Thisperiod of 1.6 ns corresponds to 8 consecutive 1 bits followed by 8consecutive 0 bits at 10 Gb/s. Case (ii) shows the results for theequivalent single section laser modulated from 20 mA to 30 mA with thesame square wave. Case (i) shows that the two-section laser is fastenough for 10 Gb/s as there is no significant impairment from rise andfall time, and as there is no turn on delay that could be associatedwith modulating, the front section from 0 mA. Comparing case (i) withcase (ii) also confirms that a larger modulated power amplitude isachieved with the two-section device for the same total currentmodulation.

A second embodiment is to replace the 2-section DFB laser with a2-section Fabry-Perot (FP) laser or distributed Bragg reflector (DBR)laser. In this embodiment the front facet or distributed reflectorcontributes to lasing of the rear section. Therefore the DC current tothe rear section needs to be high enough such that the optical poweremitted by the rear section is high enough to bleach through the frontsection as to provide enough reflected optical power from the frontfacet or front distributed reflector. In some embodiments the DBR lasermay have a long waveguide including a short active region, for examplewith lengths of 600 microns and 100 microns, respectively.

A third embodiment uses a three-section phase-shifted DFB laser withsymmetric modulation, for example, as illustrated in FIG. 15. Tomaintain the single mode selectivity of the phase shifted DFB, thedevice has anti-reflection coatings on both facets, and is separatedinto three sections, two outer sections 1510 and 1520 and a middlesection 1530. A current to the middle section is fixed above its lasingthreshold. The two outer sections and facets are generally not essentialin achieving lasing. The two outer sections are modulated in parallelfrom zero current to a modulation current that is split equally betweenthe two outer sections. As the sections are electrically isolated andthe modulated sections generally do not need a DC pre-bias current,there is no need for bias tees and the number of passive components inthe circuit connecting the laser diode driver and the laser diode issignificantly reduced.

A drawback of the third embodiment is that due to symmetry the poweremitted by each facet is equal, while typically only the power out ofone facet is collected and coupled to a fiber. Hence this is not a veryefficient way of generating a optically modulated signal. This drawbackcan be addressed in a fourth embodiment, which is a variation of thethird embodiment where the outer sections are not symmetric, and themodulation current is not split evenly between the two outer sections asto increase the modulated power out of one of the facets and improvingthe modulation efficiency of the laser.

A drawback of the embodiments one, three and four discussed immediatelyabove, is the wavelength chirp that can result from using multi-sectionlasers in this manner. While the laser wavelength chirp generated inembodiments one, three and four is acceptable for relatively short linkdistances, for longer distances the laser wavelength chirp andassociated fiber dispersion can result in significant link performancedegradation. A fifth embodiment improves the wavelength chirpperformance and is a variation on the third embodiment with two outersections 1611, 1613 modulated in a push-pull configuration by a driver1615 with differential output as shown in FIG. 16. In this particularembodiment the frequency chirp performance of the laser can be greatlyreduced, as the lasing condition does not change with time. Thepush-pull modulation shifts the laser power from one side of the deviceto the other side without changing the laser threshold condition. Insome embodiments this may be considered to be modulating the outersections from a zero current, with elimination of bias tees and a largenumber of passive components in the circuit connecting the laser diodedriver to the laser diode.

A sixth embodiment is a variation on the first embodiment where thelaser diode driver is split into an external stage 1711 driving anamplifying stage 1713 inside the laser package as illustrated in FIG.17. This embodiment results in lower overall electrical powerdissipation as the final highest power amplification stage is co-locatedwith the laser and therefore only drives current into the laser chipwhich has lower impedance than a typically used matched transmissionline interface. This particular embodiment does use a few extra passivecomponents collocated with the laser for proper RF performance. Bymodulating the current in the front section from zero, in someembodiments no bias is needed on the input of the amplifier stageco-located with the laser diode.

A seventh embodiment is a variation on the sixth embodiment where aninternal amplifier stage 1811 is connected in parallel with the frontlaser section, as illustrated in FIG. 18. This embodiment also leads toreduced electrical power dissipation with some additional passivecomponents co-located with the laser. As in the sixth embodiment, insome embodiments no bias is needed on the input of the amplifier stage,however in this case the internal amplifier stage shunts current awayfrom the front section and hence the front section can be modulated froma non zero current for a 0 bit.

An eighth embodiment is a combination of the fifth, sixth and seventhembodiment where the push-pull modulation of the laser is performedthrough a pair of internal gain stages 1911, 1913 as illustrated in FIG.19. In this configuration a DC pre-bias can be applied to the inputs ofthe internal amplification stage without bias tees, as the modulationcurrent uses a differential termination, while the DC pre-bias is in thecommon mode. Hence FIG. 19 illustrates a differential matchedtermination with high common mode input impedance. As the internalamplification stages use a voltage DC pre-bias, and not a current DCpre-bias, this DC pre-bias can be applied using high value resistorsthat do not provide a differential load to the modulation.

A ninth embodiment is a variation on all previous embodiments using alaser driver that provides small adjustable DC pre-bias on the outputlines without use of bias tees, allowing for some small pre-bias on themodulated section, which can improve the dynamic performance of thelaser in certain cases.

Aspects of the invention therefore include directly modulated lasers inwhich modulation current is provided to only portions of the laser andmethods and circuits for providing modulation current. Although theinvention has been discussed with respect to various embodiments, itshould be recognized that the invention includes the novel andnon-obvious claims supported by this disclosure and the insubstantialvariations of same.

1. A distributed feedback (DFB) laser, comprising: a waveguide includingan active region, with a grating in or about the waveguide, thewaveguide having a longitudinal length in a direction between a frontfacet and a rear facet, the waveguide being generally about layersforming a p-n junction, and an anode electrical contact and a cathodeelectrical contact; with at least one of the anode electrical contactand the cathode electrical contact having a longitudinal length in thedirection between the front facet and the rear facet less than thelongitudinal length of the active region; wherein the at least one ofthe anode electrical contact and the cathode electrical contact having alongitudinal length in the direction between the front facet and therear facet less than the longitudinal length of the active regionextends along a front portion of the longitudinal length, the frontportion being towards the front facet; wherein the at least one of theanode electrical contact and the cathode electrical contact having alongitudinal length in the direction between the front facet and therear facet less than the longitudinal length of the active region is theanode electrical contact; and further comprising a further anodeelectrical contact, the further anode electrical contact having alongitudinal length in the direction between the front facet and therear facet less than the longitudinal length of the active region, withthe further anode electrical contact extending along a rear portion ofthe longitudinal length, the rear portion being towards the rear facet.2. The DFB laser of claim 1, wherein one of the anode electrical contactand the further anode electrical contact comprises a modulation currentcontact and the other of the anode electrical contact and the furtheranode electrical contact comprises a bias current contact.
 3. The DFBlaser of claim 2, further comprising electrical isolation limitinglongitudinal movement of current from the modulation current contact andthe bias current contact.
 4. The DFB laser of claim 3 wherein theelectrical isolation comprises an isolation implant.
 5. The DFB laser ofclaim 3 wherein the electrical isolation comprises a trench.
 6. Adistributed feedback (DFB) laser, comprising: a waveguide including anactive region, with a grating in or about the waveguide, the waveguidehaving a longitudinal length in a direction between a front facet and arear facet, the waveguide being generally about layers forming a p-njunction, and an anode electrical contact and a cathode electricalcontact; with at least one of the anode electrical contact and thecathode electrical contact having a longitudinal length in the directionbetween the front facet and the rear facet less than the longitudinallength of the active region; and wherein the grating includes a phaseshift longitudinally under the at least one of the anode electricalcontact and the cathode electrical contact having a longitudinal lengthin the direction between the front facet and the rear facet less thanthe longitudinal length of the active region.
 7. A method of operating alaser, comprising: providing a data current signal into a firstlongitudinal portion of the laser and not providing the data currentsignal into a second longitudinal portion of the laser; and providing abias current signal into the second longitudinal portion of the laserand not providing the bias current signal into the first longitudinalportion of the laser.
 8. The method of claim 7 wherein the firstlongitudinal portion of the laser is a high optical power portion of thelaser and the second longitudinal portion of the laser is not a highoptical power portion of the laser.
 9. The method of claim 7 wherein thefirst longitudinal portion of the laser is about a front of the laserand the second longitudinal portion of the laser is about a rear of thelaser.
 10. The method of claim 7 wherein the laser is a distributedfeedback laser.
 11. The method of claim 7 wherein the laser is aFabry-Perot (FP) laser.
 12. The method of claim 7 wherein the laser is adistributed Bragg (DBR) reflector laser.
 13. The method of claim 7,further comprising providing the data current signal to a thirdlongitudinal portion of the laser.
 14. The method of claim 13 whereinthe first longitudinal portion of the laser is about a front of thelaser, the second longitudinal portion of the laser is about a center ofthe laser, and the third longitudinal portion of the laser is about arear of the laser.
 15. The method of claim 14 wherein the data currentsignal is a differential signal, with a first signal of the differentialsignal provided to the first longitudinal portion of the laser and asecond signal of the differential signal provided to the thirdlongitudinal portion of the laser.
 16. The method of claim 15 whereinthe second signal of the differential signal is the complement of thefirst signal of the differential signal.
 17. The method of claim 15wherein the first current signal is provided by a driver, and the driverincludes an external stage driving an amplifying stage within a packagecontaining the laser.
 18. The method of claim 17 wherein the amplifyingstage is connected in parallel with the first longitudinal portion ofthe laser.
 19. A method of operating a laser, comprising: providing adifferential signal to a pair of gain stages within a package containingthe laser, the differential signal providing a data signal, the pair ofgain stages providing a signal to a front portion of the laser and/or arear portion of the laser depending on state of the differential signal;and providing a bias signal to a central portion of the laser.
 20. Alaser and drive circuitry, comprising: a laser with a first electricalcontact and a second electrical contact, each for provision of currentsignals to the laser, and a contact coupled to ground; a dc currentsource coupled to the first electrical contact; a data signal drivercoupled to the second electrical contact; wherein the laser includes athird electrical contact for provision of current signals to the laser,with the data signal driver coupled to the third electrical contact;wherein the third electrical contact is longitudinally near the rearfacet, the second electrical contact is longitudinally near the frontfacet, and the first electrical contact is longitudinally between thethird electrical contact and the second electrical contact; and whereinthe data signal driver is configured to provide a differential signalover a differential signal line.
 21. The laser and drive circuitry ofclaim 20 wherein a first line of the differential signal line is coupledto the third electrical contact and a second line of the differentialsignal line is coupled to the second electrical contact.
 22. The laserand drive circuitry of claim 21 further comprising an amplifying stageincluding a pair of gain stages, wherein a first gain stage of the pairof gain stages is coupled to the first line of the differential signalline and the second contact and a second gain stage of the pair of gainstages is coupled to the second line of the differential signal line andthe third contact.